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Readout electronics consist of the following:
1.
Differential signal amplification.
2.
Additional amplification and signal filtering.
3.
Analog-to-Digital (A/D) converter and Clock
circuit.
4.
Battery
and
support
circuitry,
voltage
regulators, power supplies, etc.
5.
Microcomputer and support circuitry.
6.
Display board.
7.
Printer assembly.
8.
Printer interface board.
9.
Printer controller board,
4.21
DIFFERENTIAL SIGNAL AMPLIFICATION
The weight dependent output signal produced by the
load cell transducers
in the weighing
frame
is a
"differential
signal",
meaning
it
is
the
voltage
difference
between
the "+ Signal" and
"- Signal"
leads.
Integrated
circuit
U4,
an
instrumentation
amplifier, is used to interface to this differential signal
and amplify it.
The output signal from the load cells is applied to the
protection
network
cansisting
of
diodes
CRA4/CRS/CR6/CR7.
These
diodes
prevent
destructive overvoltages caused by static discharges
from damaging U4, A high frequency filter, formed by
L1/i2/C9/C10 couples the weight signal to the input
of U4.
In U4 the differential signal is amplified by a
factor of 100, and converted to a "ground-referenced"
voltage for further processing.
Capacitors C16/C17/C24 provide local bypassing of
the power supplies used by instrumentation amplifier
U4.
Capacitor C18 furnishes compensation of U4 by
reducing amplification at higher frequencies.
4.22
ADDITIONAL
AMPLIFICATION
AND
SIGNAL FILTERING
Components for an optional second gain stage may
be added on the printed circuit board.
If included
operational amplifier US is used to provide additional
gain and signal filtering.
U5, together with capacitors.
C14/C15 and resistors R17/R18, forms an active low-
pass filter.
This helps to remove fluctuations in the
weight signal caused by movement of the patient on
the scale.
US, like U4,
is "chopper-stabilized" to
correct internal offset and drift errors.
Resistors R24/R25
(optional) are used to increase
gain; resistors R26/R27 (optional) are used for gain
reduction.
An
additional
low-pass
filter stage
is
furnished by resistor R34 and capacitor C23.
4.23
ANALOG-TO-DIGITAL (A/D) CONVERSION
Integrated circuit U6 is the analog-to-digital converter.
Included
on
this
integrated
circuit
are
auto-zero
functions,
auto-polarity, and the digital and analog
functions necessary to perform dual slope integration
conversion to 20,000 counts (4% digits}.
The weight
signal voltage is applied to the analog input (pin 10) of
Us.
À reference voltage for the conversion is applied to pin
2 of U6.
The reference voltage, nominally 1 Volt, is
derived
from
the
load
cell transducer
excitation
voltage, by the divider network consisting of resistors
R29, R30, and potentiometer P1.
Adjusting P1 sets
the "span" or weight calibration of the scale.
The system clock, applied at pin 22 of U6, is used to
AID CONVERTER TIMING DIAGRAM
precisely time and control the phases of the dual slope
conversion process.
Refer to the converter timing
diagram when reading the following description.
18
5002, 5102, 5602, 6002, 6102, 6202, 6502, 6702
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